The first message sent using the new technique pays tribute to the original photographic microdots used in World War Two. It reads: "June 6 Invasion: Normandy".

"Masterpiece of espionage"

The researchers proved the DNA microdot works by pasting the tiny dots over the full stops in a typed letter, posting it and then analysing the dots when it arrived back. The message was received, loud and clear.

Spies of the future may need biotechnology expertise

In 1946, J Edgar Hoover, then director of the FBI, described microdots as "the enemy's masterpiece of espionage".

Catherine Taylor Clelland, at Mount Sinai School of Medicine, New York, told BBC News Online that the DNA microdot team had not yet been approached by the FBI.

"But we did wonder if we would get security clearance to have the paper published in the first place," she said.

Dr Taylor Clelland believes that rise of genetic engineering in plants and animals could see another use for the technique: incorporating a genetic "watermark" within the organisms themselves. "It would remove all counterfeiting."

Message in a marker

The first step of the technique is to use a simple code to convert the letters of the alphabet into combinations of the four bases which make up DNA. Next a piece of created.

Next, a piece of DNA spelling out the message is synthetically created. It contains the secret message in the middle, plus short marker sequences at each end.

This is slipped into a normal piece of human DNA.

Secret messages can now be written in the code of life

The secret message DNA strand is then mixed with ordinary DNA strands of similar length. The resulting mixture is dried on to paper which can then be cut into tiny dots. Only one strand in every 30 billion contains the message, making finding the message a fiendishly difficult task.

"To try and identify it within that complexity, when all the strands appear absolutely identical would be, we think, virtually impossible," says Dr Taylor Clelland.

The key to unravelling the message is knowing what the markers at each end of the DNA message are. These allow the message recipient to use a standard biotechnology technique, the polymerase chain reaction, to multiply only the DNA which contains the message.

This DNA can then be sequenced and the message read.

The team included Professor Carter Bancroft and Viviana Risca and is published in the journal Nature.